Devices including metal laminate with metallurgical bonds and reduced-density metal core layer

ABSTRACT

A stiff, lightweight metal laminate includes a first continuous metal layer, a second continuous metal layer, and a reduced density metal core layer disposed between the first and second continuous metal layers. The reduced density metal core layer comprises a core metal and has an average density that is less than the density of the core metal. Planar metallurgical bonds secure the first and second continuous metal layers to the reduced density metal core layer. The metal laminate may be manufactured by press rolling the reduced density metal core layer sandwiched between the two continuous metal layers, after removing or overcoating the native oxide layer on each layer surface that contacts another layer in the metal laminate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/879,831, filed Oct. 9, 2015, now U.S. Pat. No. 9,889,632, whichclaims priority to U.S. Provisional Patent Application Ser. No.62/061,824, filed Oct. 9, 2014, and to U.S. Provisional PatentApplication Ser. No. 62/181,436, filed Jun. 18, 2015. These applicationsare incorporated herein by reference in their entireties.

BACKGROUND

The present disclosure relates to stiff sheet materials usable to reducethe weight of stamped metal components, and particularly to a metallaminate sheet material.

The need for lightweight materials is widespread across many industries.In the automotive and aerospace industries, lightweight materials aredesired for improved fuel economy. In consumer recreational equipment,lightweight sheet material is desired for enhanced performance. Inconsumer electronics and mobile devices, using lightweight sheetmaterial for device housings and other components enhances portabilityand provides a lighter device. More generally, manufacturers in suchindustries make efforts to reduce the weight and thickness ofcomponents, while still selecting materials with high specific strengthand specific stiffness values and/or with other desired properties.

The use of composite materials continues to expand as a pathway towardlightweight materials. These may include carbon or glass fibercomposites, metal matrix composites, honeycomb core materials or variousother common configurations. However, composite material solutionstypically have a high cost of manufacturing. For carbon fiber, thisincludes the cost of the lay-up and impregnation of the matrix material.Manufacture of metal matrix composites typically require powder metalprocessing in relatively small batches. The high cost of manufacturingof these composite materials is an impediment to their large scaleadoption.

Further, conventional, low cost, metal forming processes such asstamping and forming from a metal coil, which are used for many highvolume components, are sometimes incompatible with composite materials.

Thus, there is an ongoing need to provide light, stiff, easily formedsheet materials for use in applications calling for lightweight butstiff sheet materials, and/or which are compatible with existingmetalworking infrastructure for stamping and forming of metalcomponents.

SUMMARY OF THE DISCLOSURE

The disclosure of this application is to be considered in its entirety,i.e. as a whole, including all text and drawings.

Disclosed in various embodiments are metal laminates comprising: a firstcontinuous metal sheet; a second continuous metal sheet; and a reduceddensity metal core layer disposed between the first continuous metalsheet and the second continuous metal sheet, the reduced density metalcore layer comprising a core metal and having an average density that isless than the density of the core metal; and a planar metallurgical bondsecuring the first continuous metal sheet to the reduced density metalcore layer; and a planar metallurgical bond securing the secondcontinuous metal sheet to the reduced density metal core layer.

The core metal may be aluminum, copper, titanium, stainless steel,carbon steel, or an alloy thereof. The first continuous metal sheet maycomprise aluminum, copper, titanium, carbon steel, stainless steel, oran alloy thereof; and the second continuous metal sheet may comprisealuminum, copper, titanium, carbon steel, stainless steel, or an alloythereof.

In specific embodiments, the core metal is aluminum, the firstcontinuous metal sheet is a stainless steel, and the second continuousmetal sheet is a stainless steel.

The thickness of the reduced density metal core layer may be at least50% of the total thickness of the metal laminate. The average density ofthe reduced density metal core layer may be between 10% and 75% of thedensity of the core metal. In more specific embodiments, the averagedensity of the reduced density metal core layer is 50% or less of thedensity of the core metal. The reduced density metal core layer maycomprise a layer of the core metal having through holes passing throughthe layer. The through holes may be asymmetric. The through holes canhave a size that is at least as large as the thickness of the reduceddensity metal core layer. The through holes may have a size that is atleast twice as large as the thickness of the reduced density metal corelayer. In more particular embodiments, the reduced density metal corelayer comprises one of (1) a layer of the core metal having throughholes passing through the layer, (2) a woven or welded wire mesh orscreen of the core metal, and (3) a porous layer of the core metal.

The reduced density metal core layer can comprise a stack of two or morereduced density metal core layers. The stack of two or more reduceddensity metal core layers may comprise reduced density metal core layerswith different geometries of through holes, woven or welded meshes orscreens, or porosities. The stack of two or more reduced density metalcore layers can comprise reduced density metal core layers of differentcore metals.

In specific embodiments, the planar metallurgical bonds of the metallaminate do not include a brazing or soldering material. In addition, ineven more particular embodiments, the planar metallurgical bond securingthe first continuous metal sheet to the reduced density metal core layerdoes not include a native oxide layer of a surface of the firstcontinuous metal sheet and does not include a native oxide layer of asurface of the reduced density metal core layer; and the planarmetallurgical bond securing the second continuous metal sheet to thereduced density metal core layer does not include a native oxide layerof a surface of the second continuous metal sheet and does not include anative oxide layer of a surface of the reduced density metal core layer.

In some embodiments, the planar metallurgical bond securing the firstcontinuous metal sheet to the reduced density metal core layer is formedby a process including (1) removing or overcoating a native oxide layeron a surface of the first continuous metal sheet and removing orovercoating a native oxide layer on a surface of the reduced densitymetal core layer and (2) press rolling the first continuous metal sheetand the reduced density metal core layer; and the planar metallurgicalbond securing the second continuous metal sheet to the reduced densitymetal core layer is formed by a process including (1) removing orovercoating a native oxide layer on a surface of the second continuousmetal sheet and removing or overcoating a native oxide layer on asurface of the reduced density metal core layer and (2) press rollingthe second continuous metal sheet and the reduced density metal corelayer.

The press rolling of the first continuous metal sheet and the reduceddensity metal core layer and the press rolling the second continuousmetal sheet and the reduced density metal core layer may be performedsimultaneously as a press rolling the first continuous metal sheet andthe reduced density metal core layer and the second continuous metalsheet with the reduced density metal core layer sandwiched between thefirst and second continuous metal sheets.

The metal laminate may include a pattern formed on an outer surface ofthe laminate that is embossed or imprinted from a pattern ofthrough-holes of the reduced density metal core layer.

The metal laminate may comprise a laminate coil.

Also disclosed herein are methods of manufacturing a metal laminate,comprising: press rolling a metal laminate including a reduced densitymetal core layer sandwiched between two outer continuous metal layerswherein the reduced density metal core layer comprises a core metal andhas an average density that is less than the density of the core metal;and prior to the press rolling, removing or overcoating the native oxidelayer on each surface of the reduced density metal core layer and of thecontinuous metal sheets that contacts another layer in the metallaminate.

The press rolling may produce a rolling reduction in the thickness ofthe metal laminate that is 50% or lower, or that is 25% or lower. Thepress rolling may produce a rolling reduction in the thickness of thereduced density metal core layer that is 60% or lower, or that is 40% orlower.

The reduced density metal core layer may include through holes providingthe reduced density metal core layer with the average density that isless than the density of the core metal. The press rolling should noteliminate the through-holes of the reduced density metal core layer.

The pressure applied in the press rolling may be effective to emboss orimprint a pattern of the through-holes of the reduced density metal corelayer onto an exterior surface of the metal laminate.

The press rolling may metallurgically bond the two outer continuousmetal layers to the reduced density metal core layer.

The operation of removing or overcoating the native oxide layer on eachsurface of the reduced density metal core layer and of the continuousmetal sheets that contacts another layer in the metal laminate maycomprise removing the native oxide layer on each said surface. This canbe done using a sputtering process.

The methods disclosed herein can be performed in a sealed enclosure thatencloses both press rollers used in the press rolling and surfaceactivation devices used in the removing or overcoating of the nativeoxide layer on each said surface.

The methods may further comprise taking up the metal laminate on atake-up roll to arrange the metal laminate as a laminate coil. Themethods do not necessarily include performing soldering or brazing inthe manufacture of the metal laminate.

Also disclosed herein are metal laminates or laminate coils manufacturedusing the methods disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same. Unlessotherwise indicated, the drawings are diagrammatic and not necessarilydrawn to scale.

FIGS. 1 and 2 show schematically a top and side view, respectively, of alightweight metal laminate structure with a reduced-density metal corelayer. In FIGS. 1 and 2, layers are removed successively moving fromleft to right to reveal underlying layers in the top view of FIG. 1.

FIG. 3 is a detail, shown in top view, of one geometry option for thereduced-density metal core layer of the laminate structure of FIGS. 1and 2.

FIG. 4 illustrates a suitable manufacturing system for manufacturing themetal laminate structure of FIGS. 1 and 2 using illustrative claddingprocessing to form a metallurgical bond between the layers.

FIG. 5 shows a perspective view of a laminate roll of the laminate ofFIGS. 1 and 2, suitably manufactured as a roll using the system of FIG.4.

FIGS. 6A-6D show schematic top views of four different through-holeconfigurations for the reduced-density metal core layer of the laminateof FIGS. 1, 2, and 5.

FIGS. 7 and 8 are photographs of the surface (FIG. 7) and cross-section(FIG. 8) of an actually fabricated laminate as described herein.

FIGS. 9 and 10 show top views of illustrative reduced-density metal corelayers in which density-reducing openings define a selected pattern.

FIG. 11 shows a top view of another illustrative reduced-density metalcore layer in which density-reducing openings comprise through-slitsoriented to provide anisotropic properties for the metal laminatestructure.

FIG. 12 presents a Load-Deflection Curve measured as described hereinfor the perforated core sample of FIGS. 1 and 2.

FIGS. 13 and 14 schematically show side views of two laminatestructures, each of which includes a stack of reduced-density metal corelayers.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure may be understood more readily by reference tothe following detailed description of desired embodiments and theexamples included therein. In the following specification and the claimswhich follow, reference will be made to a number of terms which shall bedefined to have the following meanings.

Although specific terms are used in the following description for thesake of clarity, these terms are intended to refer only to theparticular structure of the embodiments selected for illustration in thedrawings, and are not intended to define or limit the scope of thedisclosure. In the drawings and the following description below, it isto be understood that like numeric designations refer to components oflike function.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

The term “comprising” is used herein as requiring the presence of thenamed component and allowing the presence of other components. The term“comprising” should be construed to include the term “consisting of”,which allows the presence of only the named component, along with anyimpurities that might result from the manufacture of the namedcomponent.

Numerical values should be understood to include numerical values whichare the same when reduced to the same number of significant figures andnumerical values which differ from the stated value by less than theexperimental error of conventional measurement technique of the typedescribed in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 grams to 10grams” is inclusive of the endpoints, 2 grams and 10 grams, and all theintermediate values). The endpoints of the ranges and any valuesdisclosed herein are not limited to the precise range or value; they aresufficiently imprecise to include values approximating these rangesand/or values.

The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context. When usedin the context of a range, the modifier “about” should also beconsidered as disclosing the range defined by the absolute values of thetwo endpoints. For example, the range of “from about 2 to about 10” alsodiscloses the range “from 2 to 10.” The term “about” may refer to plusor minus 10% of the indicated number. For example, “about 10%” mayindicate a range of 9% to 11%, and “about 1” may mean from 0.9-1.1.

In some embodiments disclosed herein, a multilayer metal structure has aperforated (e.g., porous or expanded) metal core and metal continuousoutside sheets. More generally, the metal core layer is areduced-density metal core layer, in which the density is reduced ascompared with an equivalent layer of the same metal that does notinclude the perforations, porosity, or other spaces/vacancies that serveto reduce layer density. (In this context, “density” is to be understoodas the average density over a volume of the metal core, i.e. the ratioof core layer mass divided by core layer volume.) The multilayer metalstructure provides high strength and stiffness, while reducing componentweight. A metallurgically bonded metal sheet material comprises areduced-density metal core layer of substantially uniform thickness,which in some embodiments is less than 3 millimeters although higherthicknesses are contemplated, and two continuous metal sheetsmetallurgically bound to the major surfaces of the porous metal core. Insome embodiments, each metal sheet is less than 0.5 millimeters inthickness, although greater thicknesses are contemplated, and moreoverthe two continuous metal sheets may in general have differentthicknesses.

In such a composite sheet, the continuous metal sheets may be secured tothe reduced-density metal core layer by techniques such as brazing,soldering, or using an adhesive. However, the bond formed by suchtechniques includes an intermediate layer (the braze material, soldermaterial, or adhesive) which limits structural integrity—accordingly,the composite sheet formed using brazing, soldering, or adhesive may beincompatible with metalworking techniques such as deep drawing thatgenerate large strain. Brazes and adhesives are also have significantthickness, and are usually applied in a liquid or molten state that canbe difficult to apply uniformly in a large-scale processes, especiallyreel-to-reel processes. A metallurgical bond, that is, a bond at theatomic level with no intervening bond material and no intervening oxidelayer, has good structural integrity and is compatible with metalworkingprocesses that introduce high strain. A metallurgical bond is also“dry-formed”, that is, does not involve depositing a liquid or moltenmaterial onto the surfaces. However, obtaining a metallurgical bond overa planar interface is difficult.

One technique which can form a planar metallurgical bond is cladding, inwhich two metal sheets are pressed together under high pressure, usuallybetween rollers. Cladding breaks the surface oxide and forms ametallurgical bond between the two sheets. However, in order to overcomethe intrinsic oxide layers on the metal surfaces a high pressure must beapplied, resulting in a high rolling reduction (i.e. compressive plasticdeformation manifesting as a reduction in layer thickness and elongationtransverse to that thickness). In the case of a reduced-density metalcore layer, the high cladding pressure may compress or deform thereduced-density metal core layer to such an extent that its (average)density is substantially increased, and/or its structural integritycompromised, thereby abrogating its beneficial characteristics.

In some embodiments disclosed herein, a metallurgically bonded metalsheet comprises a porous metal core, having a multiplicity of thru holessurrounded by a matrix of metal (or, said another way, a metal layerwith thru holes), having a substantially uniform thickness of less than3 millimeters and having the surface area of the matrix on the majorsurfaces of the core being less than one half of the surface area of asheet of the same size provides sufficient strength. A porous metal corehaving a substantially uniform pattern of holes surrounded by shortsegments of continuous metal matrix provides a suitable core for ametallurgically bonded metal sheet material having advantageousmechanical characteristics and reduced weight when thin sheets aremetallurgically bound to the core.

In some embodiments disclosed herein, a laminate including areduced-density metal core layer cladded by continuous metal sheets ismade by the following operations. The reduced-density metal core layeris provided, which has a first major surface, a second major surface anda multiplicity of thru holes, pores, or other density-reducing features.Two continuous metal sheets are also provided, each having two majorsurfaces and, in some embodiments, a thickness of less than 0.5millimeters. At least one of the major surfaces of each of the metalsheets is activated. Both of the major surfaces of the reduced-densitymetal core layer are activated. The metal sheet activated surfaces arebrought into contact with the activated major surfaces of thereduced-density metal core layer and the metal sheets aremetallurgically bonded onto the reduced-density metal core layer (i.e.,with the reduced-density metal core layer sandwiched between thecontinuous metal sheets) in a rolling operation with a rolling reductionof from 0.1% to 50% reduction (that is, a reduction of 0.1%-50% in thetotal thickness of the stack produced by the rolling operation). In somepreferred embodiments, the rolling reduction is 25% or less reduction intotal stack thickness. The rolling operation causes the metal sheets andthe reduced-density metal core layer to be metallurgically boundtogether. The planar metallurgical bonds thereby formed are free oforganic adhesive, brazed metal, or solder (i.e. none of these three arepresent). In some illustrative embodiments, the activation of the metalsheet surfaces and the surfaces of the reduced-density metal core layeris preferably achieved by sputtering.

In general, the activation of the surfaces entails removal ofcontaminants that are natively present on the metal surfaces due toexposure to air. For example, a native surface oxide is present due toair exposure. Other contaminants such as various hydrocarbons may alsobe present on the air-exposed metal surfaces. Activation of the metalsheet and reduced-density metal core layer surfaces entails removingthese contaminants, especially the native surface oxide. Sputtering isone suitable approach, while other contemplated surface activationapproaches for producing clean, oxide-free, metal surfaces includethermal removal (heating to a temperature sufficient to expel thesurface oxide). In other surface activation processes, the surface oxideand/or other surface contamination is not removed, but rather isovercoated with a thin coating of a pristine metal layer with thicknessof typically 10 micrometers (μm) or less, and more preferably athickness of 2 μm or less, using a physical vapor deposition process(e.g. sputter deposition, vacuum evaporation, or so forth). Theovercoated pristine metal layer may be the same type of metal as themetal sheet or may be a different metal. For overcoating the surface ofa stainless steel sheet, the overcoating pristine metal layer may, byway of non-limiting example, be a nickel, aluminum, iron, copper, ortitanium metal layer. In addition to being thin, the surface of theovercoated pristine metal layer should not have an oxide layer. Moregenerally, the surface activation (whether by removal of the nativeoxide or overcoating) is performed in a controlled ambient, e.g. vacuumor inert gas, in order to prevent rapid re-oxidation of the activatedsurfaces.

By performing the cladding of the continuous metal sheets onto thereduced-density metal core layer after surface activation includingoxide removal in a controlled atmosphere (e.g. inert or vacuum), thecladding pressure can be substantially reduced as compared withconventional cladding processes that must break the oxide layer in orderto form the metallurgical planar surface bonds. The reduced claddingpressure enables the reduced-density metal core layer to better retainits reduced density by reducing or eliminating compressive plasticdeformation of the pores, through-holes, or other density-reducingfeatures. A light-weight laminate material with metallurgical bonds isthereby obtained, which is structurally robust and suitable formetalworking processes such as deep drawing that introduce largestrains.

On the other hand, depending upon the applied cladding pressure, thesize and shape of the through-holes or other density-reducing featuresof the reduced-density metal core layer, the thicknesses, stiffness, orother properties of the cladding continuous metal sheets, and possiblyother factors, a consequence of the bonding of the reduced-density metalcore layer and continuous sheet materials may be that the resultinglaminate sheet exhibits a surface pattern corresponding to the patternof through-holes or other openings of the reduced-density metal corelayer. Such patterning may be due to slight deformation (i.e. slight“bulging”) of the continuous metal sheets into the through-holes of thereduced-density metal core layer, or due to a surface texture change dueto differential pressures of the bonding rolls on the laminate surfaceover a thru-hole vs. over the matrix. Without being limited to anyparticular theory of operation, it is believed that during cladding thepressure exerted over a through-hole is less than the pressure exertedin the area over the “matrix” of the core, due to very small elasticdeflections of the surface sheets. As a result, texture differences arepresent on the exposed surfaces of the cladded surface sheets that mayemboss or imprint a pattern on the surface, even if the surface is flatafter bonding (i.e. does not include measurable bulging of the surfacesheets into the through-holes). It is recognized herein that this effectmay be advantageous, for example providing ornamental surface propertiesintentionally designed into the bonded laminate composite material bychoosing an ornamental pattern for the through-holes of thereduced-density metal core layer. Such a pattern could, for example,represent the logo of a consumer electronics manufacturer using thecomposite material for the metal housing of a mobile device. In otherapplications, the surface patterning may provide surface texturing orroughness that may beneficially increase friction or provide otheruseful surface properties.

The resulting composite laminate material can be constructed withsufficient strength and stiffness to be used as a cost effectivereplacement for solid sheet metal structural components. The compositelaminate material can be light, stiff, formable by conventionalmetalworking methods, uniform in characteristics, has a low cost ofmanufacturing, and is free of organic adhesives, brazing compounds, orsolder.

A more complete understanding of the components, processes andapparatuses disclosed herein can be obtained by reference to theaccompanying drawings. The drawings are provided for the purposes ofillustrating preferred implementations of the disclosure only, and notfor the purposes of limiting the disclosure.

FIGS. 1 and 2 show top (i.e. plan) and side views, respectively, of alaminate sheet material 10 having three layers. The laminate sheetmaterial 10 has a width W indicated in FIG. 1, and a long directionindicated by arrow L. In a typical roll-to-roll manufacturing process(illustrative examples of which are described later herein withreference to FIGS. 4 and 5), the roll has the width W and extends (i.e.is “rolled”) in the long direction L. Accordingly, FIGS. 1 and 2illustrate a portion or cut length of the laminate sheet that is cutalong the direction L.

The laminate sheet material 10 includes a top metal layer 12 which is acontinuous thin sheet (i.e. layer) of metal. In one embodiment, the toplayer 12 can be in the neighborhood of 0.1 millimeters thick and is acontinuous sheet of stainless steel, although greater or lesserthicknesses are contemplated. A reduced-density metal core layer 14 isof equal width W to the top metal layer 12 and comprises shortinterconnected metal segments 20 surrounding through holes 22 (see alsodetail FIG. 3). As best seen in the detail drawing of FIG. 3, theillustrative through holes 22 are hexagonal holes so that thereduced-density metal core layer 14 has a honeycomb pattern ofthrough-holes. A honeycomb pattern is advantageous because the hexagonalopenings 22 can form a highly symmetric regular lattice, but asdescribed herein the reduced-density metal core layer 14 can have a widevariety of density-reducing openings, through-holes, ports, gapped fibermesh, or so forth. The reduced-density metal core layer 14 is fabricatedfrom a metal selected to have desired material properties. In oneillustrative example, the reduced-density metal core layer 14 isaluminum or an aluminum alloy, and is about 0.5 millimeters thick(although again this is merely an example, and greater or lesserthicknesses are contemplated). In other embodiments, the reduced-densitymetal core layer 14 can have a thickness of up to 3 millimeters(although again higher thicknesses are contemplated). In otherembodiments, the reduced-density metal core layer 14 comprises anothermetal, such as titanium, stainless steel, carbon steel, or copper.

The reduced-density metal core layer 14 can be made by die cutting,stamping or other methods to produce a metal sheet having spaces 22between metal matrix elements 20. Various processes may be employed tocreate a perforated metal by punching out spaces 22 to form a repetitivepattern of holes. Another suitable process comprises expanding a metalsheet to form the spaces 22 by cutting the metal sheet with short spacedslits and stretching the metal sheet to plastically deform the metalsheet and thereby expand these slits into through holes 22. An advantageof this expanded metal process is that all the metal in the sheet isused and no punched out waste is created. (Note that in the expandedmetal process the density of the sheet is reduced by enlarging thevolume, because the overall length is increased while the mass stays thesame so that average density=mass/volume decreases.)

Instead of employing through-holes, the reduced-density metal core layercan have other density-reducing space configurations. For example, thereduced-density metal core layer may be a woven or welded wire mesh orscreen, where density reduction is achieved by the large amount ofporosity or air spaces surrounding the woven metal fibers. Otherapproaches for fabricating the reduced-density metal core layer withreduced density compared with a continuous metal sheet of the same metalthat does not include pores, through-holes, or the like is contemplated.In some illustrative examples, the reduced-density metal core layer hasa porosity of from about 10% to about 90% by volume, as determined bythe amount of open area divided by the total area of the core. In otherwords, in these embodiments the reduced density metal core layer 14 hasan average density that is between 10% and 90% inclusive of the densityof the core metal (where “density of the core metal” is the density ofthe core metal, i.e. the density of the core metal material, which is anintensive property). In more particular embodiments, the porosity is atleast 50% (that is, the average density of the reduced density metalcore layer 14 is 50% or less of the density of the core metal). In someembodiments the porosity of the reduced density metal core layer 14 isbetween 25% and 90% (that is, the average density of the reduced densitymetal core layer 14 is between 10% and 75% inclusive of the density ofthe core metal). In some embodiments the porosity of the reduced densitymetal core layer 14 is between 50% and 90% (that is, the average densityof the reduced density metal core layer 14 is between 10% and 50%inclusive of the density of the core metal).

The laminate sheet material 10 further includes a bottom metal layer 16which is a thin continuous metal sheet (i.e. thin continuous metallayer). In an illustrative example, the top layer 12 and the bottomlayer 16 are both stainless steel and about 0.1 millimeters thick. Inthe laminate 10, the reduced density metal core layer 14 is sandwichedbetween the two outer continuous metal sheets or layers 12, 16. The twocontinuous metal sheets or layers 12, 16 form the outer “skins” of thelaminate 10, and accordingly are sometimes referred to herein as (outer)metal skins 12, 16. In other embodiments, the top layer 12 and thebottom layer 16 can have the same or different thicknesses, with athickness of up to 0.5 mm in some embodiments. Again, these are merelyillustrative examples, and other thicknesses (e.g. greater than 0.5 mm)are also contemplated. In some embodiments, the two layers 12, 16 havethe same thickness which provides a symmetry that may be advantageousfor some applications (e.g., it therefore does not matter which sheet12, 16 is employed as the “top” sheet in a structure), but this symmetryis not required.

In an illustrative embodiment with commercial value in variousindustries, the continuous metal sheets 12, 16 are stainless steelsheets while the reduced-density metal core layer 14 is an aluminum oraluminum alloy metal. In other embodiments, the top and bottom metallayers are aluminum, titanium, carbon steel, or copper. Again, the twolayers 12, 16 may be made of the same material providing possiblyadvantageous symmetry, but this is not required. It should also be notedthat it is contemplated for the reduced-density metal core layer 14 tobe made of the same material as the continuous metal sheets 12, 16.

The reduced-density metal core layer 14 is also relatively thin and, asin preferred embodiments more than half of its mass has been removed increation of the thru holes or other density-reducing vacancy features.The three layers 12, 14, 16 are metallurgically bound together at theircontacting surfaces by planar metallurgical bonds 18, i.e. the metals ofthe layers are directly bound to each other without the presence oforganic adhesives, brazing compounds or solder. The creation of thislaminate sheet material 10 employs manufacturing techniques performed ina vacuum, inert gas, or other controlled atmosphere to preventre-formation of a native surface oxide after activating the contactingmetal surfaces. (Moreover, many surface activation techniques suitablyused to activate the contacting surfaces can only be performed in acertain controlled atmosphere, e.g. a vacuum backfilled with asputtering gas or so forth). In various embodiments, the thickness ofthe core is at least 50% of the total thickness of the laminatematerial, and in some preferred embodiments is from about 50% to about95% of the total thickness of the metal laminate.

As used herein, the term “aluminum” includes pure aluminum and aluminumalloys. For example, the reduced-density metal core layer 14 maycomprise a core metal such as an aluminum alloy such as 1000 seriesaluminum alloy (i.e., an alloy containing a minimum aluminum content of99 wt %), a 2000 series alloy (i.e., an alloy containing copper), a 3000series alloy (i.e., an alloy containing manganese), a 4000 series alloy(i.e., an alloy containing silicon), a 5000 series alloy (i.e., an alloycontaining magnesium), a 6000 series alloy (i.e., an alloy containingmagnesium and silicon), a 7000 series alloy (i.e., an alloy containingzinc), or an 8000 series alloy (i.e., an alloy containing other elementsnot covered by the other series). Some specific alloys that may be ofparticular suitability as the core metal in certain industrialapplications include 2024, 5052, 6061 or 7075.

Referring now to FIG. 4, an illustrative method and apparatus forcreating the laminate sheet material 10 is described. A first feed roll30 supports a coil of a first continuous metal sheet 32 that in thelaminate 10 forms the top continuous metal layer 12. A second feed roll34 supports a coil of a second continuous metal sheet 36 that in thelaminate 10 forms the bottom continuous metal layer 16. A third feedroll 40 supports a coil of a reduced-density metal sheet 42 that in thelaminate 10 forms the reduced-density metal core layer 14. The sheet 42can be pre-perforated before coiling on the feed roll 40 or,alternatively, the sheet 42 can be a continuous metal sheet that isperforated by optional die cutting rolls 44 or the like. The firstcontinuous metal sheet 32, the second continuous metal sheet 36, and thereduced-density metal sheet 42 are each drawn from their respective feedrolls 30, 34, 40 into respective activation apparatuses. The firstcontinuous metal sheet 32 wraps around a portion of a first electroderoll 52, and a first activation device 54 is supported adjacent thefirst electrode roll 52. For example, the activation device 52 may be asputtering device in which a plasma is created between the firstelectrode roll 52 and the first activation device 54 which activates thesurface of the first continuous metal sheet 32 not engaged against thefirst electrode roller 52. This activation removes (e.g. sputters away)impurities, oxides and other impediments to further processing of thefirst continuous metal sheet 32. Some suitable sputtering devices aredescribed, for example, in Saijo et al., U.S. Pat. No. 6,150,037 andSaijo et al., U.S. Pat. No. 7,175,919, which are incorporated herein byreference. In other embodiments, the activation is by way of “burying”the oxide/impurities layer by overcoating with a suitable metal layer.Some suitable overcoating approaches are disclosed, for example, in Yanoet al., U.S. Pat. No. 6,455,172 and Marancik, U.S. Pat. No. 4,011,982which are incorporated herein by reference. Similarly, the secondcontinuous metal sheet 36 is drawn off the second feed roll 34 and wrapspartially around the second electrode roller 56, where a secondactivation device 58 supported adjacent the second electrode roller 56activates (e.g. sputters) the exposed surface of the second continuousmetal sheet 36.

The reduced-density metal sheet 42 (after passing through the diecutting rollers 44, if used) is passed between a third activation device60 and a fourth activation device 62 to activate the opposing sides ofthe sheet 42, again for example by sputtering. In the configurationshown in FIG. 4, both sides are activated at the same time as the sheet42 passes between the devices 60, 62; alternatively, this processing maybe performed serially (not shown). If the third and fourth activationdevices 60, 62 are disposed in opposition as shown, electrical contactto the metal sheet 42 can be provided through adjacent support rollersor the like (not shown).

Following activation, the metal sheets 32, 36, 42 are brought intoadjacency and pressed together between press rolls 72, 74. A controlledcompression force is applied so as to metallurgically bind them togetherin a low reduction solid state bonding process to form the laminate 10of FIGS. 1 and 2. Because the native oxide is removed or overcoated witha thin metal layer (that is itself free of native oxide) before thepress rolling, the metallurgical bonds 18 can be formed at much lowercompression force as compared with a conventional cladding process, andthe lower compression force enables cladding with reduced deformation ofthe reduced-density metal core layer 14. The compression force can beusefully quantified in terms of the plastic deformation introduced tothe material. For example, in some embodiments, the pressing force islimited to provide less than 10% change in length of the laminated sheetmaterial 10 exiting the press rolls 72, 74 when compared to the lengthof the continuous metal sheets 32, 34 and metal core material 42 fedinto the press rolls 72, 74. Additionally or alternatively, thereduction in thickness of the laminate may be used as a metric of thecompression force. As the resulting laminate 10 is intended to belightweight (that is, of low overall density), it is desired to limitthe amount of plastic compression (i.e. thickness reduction) introducedby the press rolling. In other words, it is desired to have a lowrolling reduction as measured by the reduction in thickness introducedby the press rolling process. A low rolling reduction is achieved usingthe apparatus of FIG. 4 in part because contacting activated surfacesmuch more readily form a metallurgical bond during the press rolling ascompared with contacting surfaces that have the usual native oxide. The“native oxide” is an oxide layer that inherently forms on most uncoatedor pristine metal surfaces, including surfaces of steel and aluminum,upon even very brief exposure of a metal surface to air. By removing theoxide layers of the surfaces of the layers that contact other layers inthe resulting laminate before the press rolling, the force (or pressure)that needs to be applied to form the metallurgical bonds 18 is greatlyreduced, which in turn greatly reduces the rolling reduction. In someactually performed experiments, a rolling reduction of 25% or lower isachieved for the overall laminate 10. (That is, the percentage reductionin thickness of the stack of layers introduced by the press rolling is25% or less). More generally, it is preferable for the rolling reductionto be kept to 50% or lower. These values are for the stack as a whole.Because the reduced density metal core layer 14 has through-holes,porosity, is constructed as a woven or welded wire mesh or screen of thecore metal, or otherwise has low average density as compared with thedensity of the core metal, it is expected that the rolling reduction ofthe reduced density metal core layer 14 will be greater than the rollingreduction of the stack as a whole. For example, in actually performedexperiments for which the stack as a whole exhibited a rolling reductionof 25% or lower, the rolling reduction of the reduced density metal corelayer 14 (formed with through-holes) was larger, e.g. 30%. In general,the rolling reduction of the reduced density metal core layer 14 ispreferably 60% or lower, and more preferably 40% or lower. Moreover, inembodiments in which the reduced density metal core layer 14 includesthrough-holes, the press rolling preferably does not eliminate thethrough-holes (although their dimensions may be altered by the pressrolling).

With continuing reference to FIG. 4 and with further reference to FIG.5, the metallurgically bonded laminate sheet material 10 output from thepress rollers 72, 74 is suitably spooled onto a take-up roll 80. FIG. 5illustrates a rolled laminate or laminate coil 100 comprising a roll ofthe laminate 10 having width W and including the reduced-density metalcore layer 14 clad by outer continuous metal sheets 12, 16 and boundtogether by the planar metallurgical bonds 18 (which are naturally of arolled configuration in the laminate coil 100), from which the portionof the laminate sheet 10 shown in FIG. 1 may be suitably cut.Advantageously, the metallurgical bonds 18 are strong bonds whichmaintain their integrity in the rolled configuration of the laminatecoil 100. Optionally, after metallurgically bonding the laminate, a heattreatment may be performed in strand or batch processes to drivediffusion between the contacting layers and improve interfacialadhesion.

With continuing reference back particularly to FIG. 4, the entiremanufacturing apparatus of FIG. 4 is contained within a sealed enclosure82. The sealed enclosure 82 may be a vacuum chamber that is kept at areduced pressure by operation of a vacuum pump 84. Reduced pressure isuseful in the activation process occurring adjacent the activationdevices 54, 58, 60, 62. Significantly reduced pressure also aids inpreventing oxidation of materials following activation and supports thecladding performed by the press rolls 72, 74. In addition to (or insteadof) reduced pressure, the controlled atmosphere contained in the sealedenclosure 82 may be an inert atmosphere, e.g. backfilled with an inertgas such as argon.

With brief reference to FIG. 6, the reduced-density metal core layer 14can employ various through-hole configurations. FIG. 6A illustratesagain the honeycomb through-hole pattern of the embodiment of FIGS. 1and 3 in which hexagonal openings 22 are formed in the sheet such thatthe reduced-density metal core layer 14 comprises a metal matrix 20surrounding the hexagonal openings 22. Openings with other geometriesare contemplated. For example, FIG. 6B illustrates a honeycomb structureof circular (rather than hexagonal) openings. FIG. 6C shows regularCartesian array structure of square openings. It will also beappreciated that the openings can have varying degrees of anisotropy.For example, a structure comprising anisotropic diamond-shaped openingsis depicted in FIG. 6D. Employing anisotropic openings can provide thelaminate with advantageous anisotropic properties, e.g. differentamounts of stiffness in different directions.

With reference now to FIGS. 7 and 8, sample sheets of the form oflaminate 10 were clad as substantially described with reference to FIG.4. The reduced-density metal core layer 14 employed circular openings asdiagrammatically shown in FIG. 6B. The initial 3-layer sample was0.035-inch gauge overall with 0.0063-inch Stainless 301 skins(corresponding to continuous metal sheets 12, 16) on both sides of areduced-density metal core layer comprising a 0.0224-inch perforated5052 aluminum core with 50% open area. The surface of the laminate isshown in FIG. 7, while FIG. 8 shows a cross-sectional image of thelaminate. In the vacuum cladding experiments of FIGS. 7 and 8, the coregauge (that is, the thickness of the reduced-density metal core layer)decreased from 0.032-inch to 0.0224-inch, corresponding to a 30%reduction in gauge (thickness). On the other hand, the outer continuousstainless steel cladding sheets were 0.0063-inch thick and remainedmeasured at that thickness after the vacuum cladding. Combining for allthree layers, the overall reduction in gauge (thickness) was 21.5%. Thatis, the total rolling reduction caused by the rolling operation in thisexample was 21.5% reduction in total thickness, and the rollingreduction of the reduced density metal core layer was a 30% reduction inthickness.

As seen in FIG. 7, the circular openings in the perforated core createda patterned (i.e. “dimpled”) effect on the exterior of the laminate,which is readily perceived visually. The cross-sectional view of FIG. 8shows that these dimples are due to slight protrusion or bulging of thecladding steel into the through-holes. This effect is due to thethrough-holes being large (relative to the thickness of the continuousmetal sheets 12, 16), and this surface effect can be suppressed by usinga larger number of smaller through-holes relative to the steel skinthickness so as to maintain the same (average) density for thereduced-density metal core layer. The dimpling may also be suppressed byadditionally or alternatively reducing the compression force applied bythe press rollers 72, 74. It is straightforward to optimize thiscompression force to minimize the dimpling effect while providing asufficiently strong planar metallurgical bond 18 for a givenapplication.

On the other hand, for some applications the imprinting of the patternof through-holes of the reduced-density metal core layer 14 on the outersurfaces of the continuous metal sheets 12, 16, such as that seen inFIG. 7, may be desirable for aesthetic reasons, or to increase surfacefriction, or so forth.

With reference to FIG. 9, for example, the through-holes of thereduced-density metal core layer 14 can optionally be formed in a chosenpattern, such as a “sun burst” pattern shown in FIG. 9, or the logo of amanufacturer, or the brand name of a product, or so forth. The openingsare still chosen to be of sufficient aggregate area to provide theintended reduction in average density for the reduced-density metal corelayer 14, but their specific layout is chosen to imprint a desiredpattern on the surface of the resulting laminate 10.

With reference to FIG. 10, in some embodiments the sun burst patternshown in FIG. 9 serves as a repeating unit cell of the through-holepattern of reduced-density metal core layer 14. Here in addition toadjusting the total area of the through-holes of each unit cell, thespacing between adjacent unit cells can be adjusted to achieve thedesired average density reduction for the reduced-density metal corelayer 14.

With reference to FIG. 11, in an extension of this approach, anasymmetric pattern of through-holes (e.g. through-holes of asymmetriccross-section with the long cross-section directions of thethrough-holes arranged in a generally aligned pattern, and/orthrough-holes whose spacing along one direction is smaller than alongthe transverse direction) may be employed to optimize the properties inone direction of the composite laminate 10, while keeping the overallmass to a desired light weight value. Illustrative FIG. 11 employselongated through-holes (i.e. through-slots) all oriented in the samedirection. Another example of such an anisotropic through-hole pattern,with a lower degree of anisotropy, is shown in FIG. 6D. This approachallows certain material properties of the laminate 10 to be maximized inone direction, while keeping the overall open area at a target value toachieve the desired laminate weight. For example, the anisotropicthrough-slots of FIG. 11 may provide advantageous anisotropic electricalor thermal sheet conductivity, anisotropic mechanical properties, or soforth.

TABLE 1 Tested Sample Laminate Structures 110 Skin Strips Core LayerBonded Example Material Thickness Material % Open Core Thickness No.Composition (mm) Composition Area Pattern (mm) 1 304 SS 0.062 Al 110062% Diamond 0.37 2 304 SS 0.062 Al 1100 67% Diamond 0.38 3 304 SS 0.12Al 1100 62% Diamond 0.53 4 304 SS 0.12 Al 1100 67% Diamond 0.53 5 305 SS0.16 Al 5052 50% Circles 0.90

With reference to Table 1, laminate samples were tested as to theirmechanical properties. Coils (e.g. as in the laminate coil 100 of FIG.5) were fabricated using stainless steel for the outer skin (sheet)layers and having various aluminum reduced-density metal core layerswith both circular perforations (FIG. 6B) and diamond perforations (FIG.6D) as listed in Table 1. The total bonded thickness varied from 0.37 to0.90 mm and the percentage open area in the core ranged from 50% to 67%.Example No. 5 corresponds to the sample imaged in FIGS. 7 and 8.

The effective bending modulus for a sample of laminate 10 wascharacterized experimentally utilizing well-known beam bending equationswhich link the beam deflection at an applied load to the elasticmodulus:

$E = \frac{4{Pl}^{3}}{{wt}^{3}\delta}$where P is a force applied at the distal end of a cantilevered beam, lis the beam length, w is the beam width, t is the beam thickness. Themaximum deflection of the beam, denoted δ_(max), is given by:

$\delta_{\max} = \frac{{Pl}^{3}}{3{EI}}$where

$I = \frac{{wt}^{3}}{12}$

With reference to FIG. 12, an experimental Load-Deflection Curve isshown for the perforated core sample of FIGS. 1 and 2. By fitting atrend line to the linear elastic region indicated in FIG. 12, theeffective bending modulus was calculated. That is, the linear portion ofthe Load-Deflection Curve was used to experimentally determine theflexural modulus, E. The effective bending modulus of the compositematerial of FIGS. 1 and 2 was found to be 151 GPa.

In bending, the stiffness of composite beam structures is heavilydominated by the stiffness of the materials on the top and bottomsurfaces of the beams. This flexural modulus is defined as:

$E_{Flexural} = {\frac{1}{I_{Overall}}{\sum\limits_{n}\left( {E_{n} \times I_{n}} \right)}}$where E_(n) is the elastic modulus of each layer, and I_(n) is themoment of inertia for each layer. For a three-layer sandwich structuresuch as that of the laminate 10 of FIGS. 1 and 2, the moments of inertiafor the skin layers (I_(s)) and core (I_(c)) are

$I_{s} = {{\frac{b}{12}\left( {t^{3} - t_{c}^{3}} \right)\mspace{14mu}{and}\mspace{14mu} I_{c}} = \frac{{bt}_{c}^{3}}{12}}$where t_(c) is the thickness of the core material, t is the overallcomposite thickness, and b is the width of the beam. However, for aperforated core, the effective width of the core b_(c) can be modeled asbeing reduced by a factor (1−f) to yield:b _(c)=(1−f)bwhere f is the fraction of the core material removed by perforations.Using these equations, predicted bending modulus values was comparedwith those experimentally determined from cantilever bend measurementsas previously described. These results are presented in Table 2 for theExamples of Table 1.

TABLE 2 Bending Modulus results Bending Modulus, % Open % Steel E (GPa)Example No Core per Side Meas/Calc 1 62% 15% 141/146 2 67% 15% 135/137 362% 24% 179/172 4 67% 24% 174/172 5 50% 18% 151/154Good agreement in measured and calculated flexural modulus valuesconfirm that the laminate structure 10 including the reduced-densitymetal core layer 14 provides a stiff, lightweight and formable material.

For comparison of materials for use as stiff, lightweight beams, acommon metric is used that was originally developed by Ashby. Thismetric compares materials by the metric E^(1/2)/ρ where larger valuescorrespond to superior performance as a stiff, lightweight beam. Table 3compares measured values for the example composite materials of Table 1with published values for common stamped and formed metals such asaluminum, steel and titanium.

TABLE 3 Ashby metric for Examples of Table 1 compared with literaturevalues Material % Open % Steel Modulus, E Density, ρ System Core perSide (GPa) (g/cm³) E^(1/2)/ρ Example 2 67% 15% 135 3.38 3.44 Example 550% 18% 151 3.7 3.32 Example 1 62% 15% 141 3.7 3.21 Aluminum 69 2.7 3.08Example 4 67% 24% 174 4.55 2.90 Example 3 62% 24% 179 4.7 2.85 Titanium116 4.5 2.39 Stainless 193 8 1.74 Steel

As demonstrated by the E^(1/2)/ρ values presented in Table 3, thelaminate 10 of FIGS. 1 and 2 is able to provide superior performance asa light weight bending material compared to conventional metals.Moreover, manufacturing of the composite laminate 10 is economical,using roll-to-roll processing with the resulting laminate beingcompatible with conventional metalworking such as stamping and formingdue to the strong metallurgical bonds.

The actually fabricated samples of Table 1 comprise steel continuousmetal sheets bonded over a perforated aluminum core layer. Features ofthe actually fabricated laminate material include reduced weight (e.g.,nominally 60% lighter than steel), increased stiffness (e.g., nominallytwo times stiffer than aluminum), and being fully formable in highvolume stamping processes.

While steel/aluminum structures have been fabricated as test samples,the disclosed approach is more generally applicable to other metalcombinations. Table 4 lists some other contemplated laminate structures.Table 4 is to be understood as a non-limiting list of some suitablemetal combinations for the laminate 10, and it is to be understood thatthe laminate 10 may comprise other metal combinations.

TABLE 4 Some suitable materials for the laminate structure Bending CoreSkin Modulus, E Density, ρ Skin/Core Metals Porosity Thickness (GPa)(g/cm³) E^(1/2)/ρ Aluminum/ 60% 20% 60 1.73 4.48 Aluminum Steel/Steel70% 10% 122 3.52 3.14 Steel/Aluminum 85%  5% 59 1.16 6.62 Titanium/ 60%10% 71 1.77 4.76 Aluminum

For each material combination, various alloys and materials tempers areavailable. For instance, the Aluminum/Aluminum composite of Table 4 maybe constructed from grades such as 2024, 3003, 6061 or 7075. For heattreatable aluminum grades, precipitation hardening treatments can beincluded after stamping and forming to increase the strength. Variouscombinations of metals may be used to achieve specific properties forthe laminate 10. For example, the Steel/Steel combination of Table 4 maybe used to achieve high temperature stability while also reducingcomponent weight.

In the foregoing examples, the composite laminate structure 10 wasemployed, which includes three layers: the single reduced-density metalcore layer 14; and the two cladding continuous metal sheets 12, 16.However, in order to separate the outer steel skins as far as possiblewhile maintaining a core of very low density, it may be beneficial tolayer multiple reduced-density metal core layers. Employing a stack oftwo or more reduced-density metal core layers facilitates keeping thesize of the through-holes of each layer small, while keeping the totalcore thick. There is a correlation between the accessible perforationdimension and the thickness—generally the perforation size (e.g.,measured as the diameter of through-holes of circular cross-section, oras the largest diameter of through-holes with hexagonal cross-section)should be at least 1-2 times the thickness of the core layer, andpreferably even larger. But as explained with reference to FIGS. 7 and8, through-holes of large size can lead to formation of dimples on thesurface of the laminate, which can be undesirable for some applications.By contrast, a stack of two or more reduced-density metal core layerswith thinner individual gauges would enable small perforations and largefractions of open area while keeping a thick core.

With reference to FIG. 13, a laminate is similar to the laminate 10 ofFIGS. 1 and 2 and includes the cladding continuous metal sheets 12, 16;however, the single reduced-density metal core layer 14 of theembodiment of FIGS. 1 and 2 is replaced in the embodiment of FIG. 13 bya stack of three reduced-density metal core layers 14 ₁, 14 ₂, 14 ₃.This three-layer core configuration enables wider separation of the highstrength continuous clad sheets 12, 16, which maintains a large bendingstiffness while (further) reducing density for this separation. Thestack of three reduced-density metal core layers 14 ₁, 14 ₂, 14 ₃ alsoenables smaller pore sizes relative to the overall core thickness. Itwill be appreciated that in such a multi-core layer design, the numberof reduced-density metal core layers in the stack of threereduced-density metal core layers can be other than the illustrativethree layers (e.g. may be two layers, three layers, four layers, etcetera. Providing a finer pore geometry in such a stack also helps tokeep surfaces flat.

The laminate of FIG. 13 may be fabricated using the same low reductionbonding process as previously described with reference to FIG. 4, exceptthat additional source sheet rolls may be needed. To achieve multiplelayer bonding, two or more passes through the rollers may be utilized tobuild up the larger stack of layers of FIG. 13. (Indeed, even in thethree-layer laminate 10 of FIGS. 1 and 2, it is contemplated to employtwo passes, e.g. one to form the metallurgical bond between layers 12,14 and a second pass to form the metallurgical bond between layers 14,16). By performing surface activation (whether by removal of the nativeoxide or overcoating) for each surface joining another layer (that is,activating the surface of the continuous clad layer 12 which contactsthe reduced-density metal core layer 14 ₁, and activating both surfacesof the reduced-density metal core layer 14 ₁, and activating bothsurfaces of the reduced-density metal core layer 14 ₂, and activatingboth surfaces of the reduced-density metal core layer 14 ₃, andactivating the surface of the continuous clad layer 16 which contactsthe reduced-density metal core layer 14 ₃) a strong planar metallurgicalbond is formed at each of the interfaces (that is between layers 12 and14 ₁, and between layers 14 ₁ and 14 ₂, and between layers 14 ₂ and 14₃, and between layers 14 ₃ and 16).

With reference to FIG. 14, another advantage of employing a stack ofreduced-density metal core layers is that the different layers of thestack may be made of different materials. For example, the illustrativelaminate of FIG. 14 includes a stack of three reduced-density metal corelayers, in which the outer two reduced-density metal core layers 14_(outer) are made of one metal and have a common through-hole geometry,and the inner reduced-density metal core layer 14 _(inner) is made of adifferent metal and/or has a different through-hole geometry as comparedwith the outer layers 14 _(outer). For example, the middle core 14_(inner) may be optimized for the lowest density while the outer corelayers 14 _(outer) are optimized to provide a desired imprint pattern onthe surface of the laminate (using the same mechanism as previouslydescribed with reference to FIGS. 7 and 8). The desired pattern could bea logo, image or a visually appealing surface finish. Multiple layerscould also be utilized to tailor the properties of the strip withdifferent stiffness and/or density properties in each layer.

The metal laminate can be used for diverse applications currently usingstamped metals in aerospace, automotive and consumer goods applications.Particular applications can include light weight structural panels,electrical conduit lines, bumpers or fenders, electronic enclosures, andthe like. Other uses of the composite laminate will readily occur tothose skilled in the art. The natural flame-retardant properties of themetal laminate and the temperature stability of the metallurgicalinterface between the core and the skin materials are especiallyadvantageous properties for the aerospace and automotive industries.

The disclosure has been described with respect to preferredimplementations. Modifications and alterations will occur to others upona reading and understanding of this specification. It is our intentionto include all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalence thereof.

The invention claimed is:
 1. A device comprising a component that ismade from a metal laminate, the metal laminate comprising: a firstcontinuous metal sheet; a second continuous metal sheet; a reduceddensity metal core layer disposed between the first continuous metalsheet and the second continuous metal sheet, the reduced density metalcore layer comprising a continuous metal matrix formed from a core metaland having an average density that is less than the density of the coremetal; and a planar metallurgical bond securing the first continuousmetal sheet to the reduced density metal core layer; and a planarmetallurgical bond securing the second continuous metal sheet to thereduced density metal core layer.
 2. The device of claim 1, wherein thecore metal is aluminum, copper, titanium, stainless steel, carbon steel,or an alloy thereof.
 3. The device of claim 1, wherein: the firstcontinuous metal sheet comprises aluminum, copper, titanium, carbonsteel, stainless steel, or an alloy thereof; and the second continuousmetal sheet comprises aluminum, copper, titanium, carbon steel,stainless steel, or an alloy thereof.
 4. The device of claim 1, whereinthe core metal is aluminum, the first continuous metal sheet is astainless steel, and the second continuous metal sheet is a stainlesssteel.
 5. The device of claim 1, wherein the thickness of the reduceddensity metal core layer is at least 50% of the total thickness of themetal laminate.
 6. The device of claim 1, wherein the average density ofthe reduced density metal core layer is between 10% and 75% of thedensity of the core metal.
 7. The device of claim 1, wherein the reduceddensity metal core layer comprises a layer of the core metal havingthrough-holes passing through the layer.
 8. The device of claim 7,wherein the through-holes are asymmetric.
 9. The device of claim 7,wherein the through-holes have a size that is at least as large as thethickness of the reduced density metal core layer.
 10. The device ofclaim 7, wherein the reduced density metal core layer comprises one of(1) a layer of the core metal having through-holes passing through thelayer, (2) a woven or welded wire mesh or screen of the core metal, and(3) a porous layer of the core metal.
 11. The device of claim 1, whereinthe reduced density metal core layer comprises a stack of two or morereduced density metal core layers.
 12. The device of claim 11, whereinthe stack of two or more reduced density metal core layers comprisereduced density metal core layers with different geometries ofthrough-holes, woven or welded meshes or screens, or porosities.
 13. Thedevice of claim 11, wherein the stack of two or more reduced densitymetal core layers comprise reduced density metal core layers ofdifferent core metals.
 14. The device of claim 1, wherein the planarmetallurgical bonds do not include a brazing or soldering material. 15.The device of claim 1, wherein: the planar metallurgical bond securingthe first continuous metal sheet to the reduced density metal core layerdoes not include a native oxide layer of a surface of the firstcontinuous metal sheet and does not include a native oxide layer of asurface of the reduced density metal core layer; and the planarmetallurgical bond securing the second continuous metal sheet to thereduced density metal core layer does not include a native oxide layerof a surface of the second continuous metal sheet and does not include anative oxide layer of a surface of the reduced density metal core layer.16. The device of claim 1, wherein: the planar metallurgical bondsecuring the first continuous metal sheet to the reduced density metalcore layer is formed by a process including (1) removing or overcoatinga native oxide layer on a surface of the first continuous metal sheetand removing or overcoating a native oxide layer on a surface of thereduced density metal core layer and (2) press rolling the firstcontinuous metal sheet and the reduced density metal core layer; and theplanar metallurgical bond securing the second continuous metal sheet tothe reduced density metal core layer is formed by a process including(1) removing or overcoating a native oxide layer on a surface of thesecond continuous metal sheet and removing or overcoating a native oxidelayer on a surface of the reduced density metal core layer and (2) pressrolling the second continuous metal sheet and the reduced density metalcore layer.
 17. The device of claim 16, wherein the press rolling thefirst continuous metal sheet and the reduced density metal core layerand the press rolling the second continuous metal sheet and the reduceddensity metal core layer is performed simultaneously as a press rollingthe first continuous metal sheet and the reduced density metal corelayer and the second continuous metal sheet with the reduced densitymetal core layer sandwiched between the first and second continuousmetal sheets.
 18. The device of claim 1, including a pattern formed onan outer surface of the laminate that is embossed or imprinted from apattern of through-holes of the reduced density metal core layer.
 19. Adevice component that is made from a metal laminate, the metal laminatecomprising: a first continuous metal sheet; a second continuous metalsheet; a reduced density metal core layer disposed between the firstcontinuous metal sheet and the second continuous metal sheet, thereduced density metal core layer comprising a continuous metal matrixformed from a core metal and having an average density that is less thanthe density of the core metal; and a planar metallurgical bond securingthe first continuous metal sheet to the reduced density metal corelayer; and a planar metallurgical bond securing the second continuousmetal sheet to the reduced density metal core layer.
 20. The devicecomponent of claim 19, wherein the device component is a structuralpanel, an electrical conduit line, an automotive bumper or fender, anelectronic enclosure, or a device housing.
 21. A device comprising acomponent that is made from a metal laminate, the metal laminatecomprising: a first continuous metal sheet; a second continuous metalsheet; a reduced density metal core layer disposed between the firstcontinuous metal sheet and the second continuous metal sheet, thereduced density metal core layer comprising a core metal in the form ofa continuous metal matrix surrounding through-holes, and having anaverage density that is less than the density of the core metal; and aplanar metallurgical bond securing the first continuous metal sheet tothe reduced density metal core layer; and a planar metallurgical bondsecuring the second continuous metal sheet to the reduced density metalcore layer.